1. Field of the Invention
The invention relates to a sheet feed cassette that is installed to a sheet feeder of an image forming apparatus such as a laser printer and used to feed and forward sheets toward the image transfer section one by one. More particularly, the invention is directed to a sheet feed cassette capable of holding the front end of a sheet to a predetermined position by moving the bottom plate up and down while maintaining horizontal state thereof.
2. Description of the Related Art
In an image forming apparatus such as a laser printer or an electronic copying machine, cut sheets are contained in a sheet container unit such as a sheet feed cassette or a sheet feed tray, such sheet container unit is installed to a sheet feeder of the image forming apparatus, and the sheets are fed one by one using a sheet feed member such as a sheet feed roller disposed on the sheet feed unit. The image forming apparatus using such a sheet container unit is, e.g., a laser printer shown in FIG. 8. The laser printer 20 shown in FIG. 8 employs a sheet feed cassette la as a sheet container unit. A sheet feed roller 50 is disposed on the sheet feeder to which the sheet feed cassette la is installed, and a sheet fed by the sheet feed roller 50 is forwarded by a forward roller unit 52 along a sheet forward path 51.
In the upstream side of an image transfer section from a photoreceptor drum 41, a registration member 58 is disposed. Using this registration member 58, the sheet is temporarily stopped and then forwarded at such a timing as to allow its front end to be aligned with a toner image formed on the photoreceptor drum 41. Then, the toner image is transferred onto the sheet by discharging operation of a transfer corotron 24, a copy is prepared by fusing the sheet carrying the toner image through a fuser unit 25, and the copy is discharged through a discharge roller 26 or 26a. Like ordinary electrophotographic printers, the above-mentioned laser printer 20 includes a charge corotron 45, a developing roller 43 of a developing unit, and a blade 46 of a cleaning unit with respect to the photoreceptor drum 41. Further, using a laser beam applied from an image writing unit 31, an image is written to form a latent electrostatic image on the photoreceptor drum 41, and toner is supplied from the developing unit to the latent electrostatic image to fuse and form the toner image.
Further, in the laser printer 20 shown in FIG. 8, the sheet feed cassette to be installed to the sheet feeder is provided with a means for elevating a bottom plate that supports sheets, and a means for biasing the front end portion of the plate against the sheet feed roller at a predetermined pressure while supporting the rear end portion of the plate which is the sheet feed side through a pivot. Such means for holding the sheets at an elevated position includes, as disclosed in Japanese Utility Model Unexamined Publication No. Sho. 56-6832, one that elevates the sheet supporting bottom plate so as to be inclined by a pushing-up arm, or one that elevates the bottom plate by a spring.
However, as described above, in the case of the sheet feed cassette in which the rear portion of the sheet supporting bottom plate is supported through the pivot to elevate the front end side of the plate so as to be inclined, the angle of inclination of the sheet varies depending on the amount of sheets remaining on the plate, which is a problem. When the angle of inclination of the bottom plate becomes varied, the position of the front end on the feed side of the sheet becomes inconsistent and the position at which the front end of the sheet abuts against the sheet feed roller varies, thereby imposing the additional problem that the timing of feeding each sheet cannot be maintained constant. That is, as shown in FIG. 6, the relationship between the amount of sheets contained in the sheet feed cassette and the position of the front end of each sheet at the registration position on the sheet forward path is such that the greater the amount of sheets contained in the sheet feed cassette, the more the position of the front end of the sheet at the registration unit varies. For example, in the case of a sheet feed cassette that can contain 250 sheets, a variation of 4 mm or more is observed between a case in which such cassette contains a large amount of sheets therein and a case in which it contains few sheets. To overcome this problem, the conventional image forming apparatus has to arrange a member for positioning the front end of a sheet such as a registration roller along the sheet forward path so that the sheet can be forwarded toward the image transfer section using this registration roller at a timing corresponding to a toner image formed on the photoreceptor drum.
To overcome the above problem, as proposed, e.g., in Japanese Utility Model Unexamined Publication No. Sho. 57-57954, a means for supporting the bottom plate by two springs to thereby maintain the front end on the sheet feed side horizontally. However, if the means for biasing the bottom plate upward all the time by the springs as in the conventional example is employed, the operation of inserting the front ends of the sheets into the lower portion of a snubber becomes cumbersome when replenishing sheets in the sheet feed cassette. Also, when the sheet feed cassette is extracted from the sheet feed unit, the sheets contained in the sheet feed cassette must be lowered using some kind of means. In this case, if the lowering means resisting the spring force is provided on the sheet feed unit of the image forming apparatus body, the mechanism of the sheet feed unit of the apparatus is complicated, which is a disadvantage.
In contradistinction thereto, e.g., Japanese Utility Model Unexamined Publication No. Sho. 56-64938 proposes a means for moving the bottom plate of a sheet feed tray vertically using a link mechanism. In the sheet feed tray referred to in this conventional example includes a means for driving the bottom plate using a cam mechanism, the bottom plate being supported by the link mechanism. The bottom plate is moved up and down while maintaining the horizontal state thereof, so that it can prevent the above-mentioned timing at which sheets are fed from being inconsistent. However, when using the above-mentioned drive mechanism with the link, the mechanism for supporting the plate inside the sheet feed tray becomes complicated, and a drive mechanism for the link is also complicated because the drive mechanism must be provided in the sheet feed unit. As a result, this type of sheet feed tray is not suitable for use in a small-sized image forming apparatus from the viewpoint of installation space, manufacturing cost, and other various factors.